1. Field of the Invention
This invention relates to a pigment ink to be ejected by an ink-jet recording method. More particularly, it relates to a pigment ink which is less causative of changes in particle diameter of pigment particles and viscosity of inks and has a superior storage stability, even when left at a high temperature over a long period of time.
2. Related Background Art
Inks conventionally used in ink-jet recording have commonly been those composed chiefly of water and making use of, as coloring materials, dyes which are soluble in the chief-component water. In recent years, in order to improve weatherability of recorded images on recording mediums, it is studied to use pigments as coloring materials for ink-jet recording inks. However, since the pigments are not soluble in water base inks, the pigments must stably be dispersed in inks, e.g., by the use of dispersing agents to prevent pigment particles from agglomerating or settling. It, however, is difficult to keep such a pigment dispersed stably over a long period of time. Accordingly, as stated below, various proposals have hitherto been made in order to provide pigment inks having a superior storage stability.
For example, in Japanese Patent Application Laid-open No. H03-287676, it is proposed to use as a dispersing agent of the pigment a specific anionic surface-active agent. In Japanese Patent Application Laid-open No. H08-003500, it is also proposed to use as a dispersing agent a specific water-soluble polymer. In Japanese Patent Application Laid-open No. H05-214284, an ink-jet recording ink is disclosed in which an organic solvent is added to improve dispersion stability of the pigment. In Japanese Patent Application Laid-open No. H11-172170, a proposal is made which relates to a pigment modified with a metal compound or an organic compound. In Japanese Patent Application Laid-open No. 2000-273383, a pigment is proposed in which a water-soluble pigment derivative is adsorbed on pigment particle surfaces.
Further, as in Japanese Patent Application Laid-open No. H08-209045, a proposal is made on improvement of a process for producing a resin-dispersed pigment ink. Also, according to a proposal made in Japanese Patent Application Laid-open No. 2001-262015, the surface tension, viscosity, conductivity and pH value of an ink are controlled within specific ranges to achieve dispersion stability of the pigment ink to a certain extent.
However, in recent years, there is a tendency for the particle diameter of the pigment in ink and the diameter of nozzles through which the ink is ejected, to be made smaller in order that images formed by ink-jet recording can have a higher resolution and a higher grade. Under such circumstances, in order for the pigment ink to be stably ejected, it is necessary to improve the dispersion stability and storage stability of the pigment so that the pigment can be prevented from agglomerating or settling over a much longer period of time.